1. Field of the Invention
The present invention relates to a scan conversion apparatus for converting an interlace video signal to a progressive video signal.
2. Description of the Related Art
In order to output a signal transmitted by an interlace signal such as NTSC (National Television Standard Committee) and PAL (Phase Alternation by Line) on a plasma display and a liquid crystal display which have been rapidly popularized in recent years, it is necessary to perform an interlace/progressive conversion (abbreviated as “IP conversion” hereinafter). Conventionally methods for interpolating for a pixel between lines of an interlace video signal are used for the IP conversion.
An intra-field interpolation is given as an example of a method for interpolating a pixel. FIG. 1 is a diagram for describing an outline of the intra-field interpolation according to the conventional technique. In the showing of FIG. 1, only the pixels of specific spots are clearly shown to explain interpolation process, while other zones are omitted. Among the zone clearly showing pixels, the color-distinguished lines are pixels obtained by IP-converting an interlace video signal. The white-out line in the drawing indicates an interpolated line. As an example, a pixel indicated by the x-mark in FIG. 1 shows an example of interpolation in a bi-linear interpolation by using a pixel indicated by the o-mark.
An intra-field interpolation does not change a resolution in the vertical direction from the case of no interpolation, while a line based on the actual video signal and the interpolated line are guaranteed as the video signal of the same clock time. Such an interpolation method is effective when there is a movement in the video, and there are techniques capable of performing the above described intra-field interpolation in the case of a movement existing as a result of a judgment of a presence/absence of a movement in the video, as shown in Laid-Open Japanese Patent Application Publication Nos. 2003-32636, 2004-266395, 2005-191619 and 2006-67009.
FIG. 2 is a diagram for describing a motion-adaptive IP conversion according to a conventional technique. The movement judgment unit 101 shown in FIG. 2 judges a movement of a video image based on a comparison between the video signal of the previous field and that of the current field. It then generates the inter-field interpolation pixel for a still image and generates an intra-field interpolation pixel for a motion image based on the judgment result produced by the movement judgment unit on the movement of the video image.
FIG. 3 is a diagram for describing a motion compensation IP conversion according to a conventional technique. The motion compensation IP conversion compares between field signals of the same parity, thereby measuring a movement amount of the video image and generating an interpolated pixel based on the movement amount. The motion vector detection unit 111 shown in FIG. 3 searches a movement amount of the pixels of a predetermined target between two field signals (i.e., Fld(t) and Fld(t−2) herein) and estimates a movement amount in a single field to interpolate it. The blend process unit calculates a weighted sum of inter-field/intra-field interpolation pixels for every spot within a field signal and outputs a post-IP conversion signal.
Incidentally, transmitted video signals include a film material of a movie, animation, et cetera, and a computer graphic material and such. These signals are subjected to a telecine conversion, such as 2:2 pull-down, 2:3 pull-down or such at a transmission source of the signal and transmitted by superimposing on the interlace video signal. A reception side is required to perform a process for releasing these pull-down sequences.
FIG. 4 is a diagram showing a configuration of a scan conversion apparatus used for releasing a pull-down. When a 2:3 or 2:2 video image transmission sequence is detected from a received signal at the 2:3/2:2 sequence detection unit 121, the video signal is given to the pull-down release process unit at which a process for a progressive conversion is carried out. A video signal not subjected to a pull-down is subjected to a scan conversion at the interlace-use IP conversion unit. Note that the configuration and operation of the interlace-use IP conversion unit are similar to the process of the motion-adaptive IP conversion or motion compensation IP conversion.
Here, a further description of the pull-down sequence is provided. FIGS. 5A and 5B are diagrams for describing a pull-down sequence. FIG. 5A is a drawing expressing a 2:2 pull-down process, while FIG. 5B is a drawing expressing a 2:3 pull-down.
First is a description on the 2:2 pull-down shown in FIG. 5A. In relation to the time axis shown in the drawing, the upper part is a video signal prior to executing a pull-down and the lower part is a video signal after executing the pull-down. Also in the drawing, the solid line expresses a signal transmitted in the top field among the video signal, while the dotted line expresses a signal transmitted in the bottom field.
When changing a certain frame into an interlace video signal, the 2:2 pull-down process transmits a signal by loading the top field and bottom field alternately. Here, transmission timings of individual fields are assumed to be t1, t2 and so on. Assuming that a video signal of the top field of the first frame is loaded at the clock time t1, a video signal of the bottom field of the first frame is loaded at the clock time t2 that is the next transmission timing. Likewise, a video signal of the top field is loaded at the clock time t3, and that of the bottom field is loaded at the clock time t4, for the second frame. As such, the assigned number of fields is constituted by the ratio of 2:2 between the odd numbered frame and even numbered frame.
Comparably, in the 2:3 pull-down process shown in FIG. 5B, the assigned number of fields is constituted by the ratio of 2:3 between the odd numbered frame and even numbered frame. That is, video signals of the top field and bottom field are loaded/transmitted at the clock times t1 and t2, respectively, for the first frame, while video signals of the top field, bottom field and top field are loaded/transmitted at the subsequent clock times t3, t4 and t5 for the second frame.
In actuality, a hardly avoidable problem at the detection unit of the 2:2 and 2:3 sequences is an erroneous detection due to a noise and such. Although a high image quality can be highly effectively available for a 2:2 material and 2:3 material, an erroneous detection can lead to a large degradation in the image quality. Because of this, a final adjustment at a television set manufacturer is generally set up for detecting only a material which can be securely detected. As a result, there are some video image sequences which are not detected as the 2:2 or 2:3 video image sequences among the transmitted sequences.
Meanwhile, in the actual video image signals, there is a case in which the 2:2 or 2:3 pull-down video signal and interlace video signal are temporally and spatially mixed and transmitted. In such a case, it has been impossible to partially perform a conventional interlace-use IP conversion.
Furthermore, it is difficult to detect a difference of changes between fields for a slow motion video image because the difference of change between fields is small. Because of these reasons, an interlace-use IP conversion is performed on a video image signal in which a sequence is not detected.
Performing a motion-adaptive interpolation or a motion compensation interpolation in the interlace-use IP conversion on the 2:2 or 2:3 video image signal results in generating a problem of flicker, jaggy, interlace interference, et cetera. That is, despite there is actually no need to interpolate video signals of two fields which are pulled down to the top and bottom for a telecine video signal, an interpolation process is carried out if a motion is detected between the current and next frames, causing a disturbance in the image as a result of interpolating a pixel for a signal which does not actually exist within a frame.
In the meantime, Laid-Open Japanese Patent Application Publication Nos. 2005-102191 and 2004-180242 have proposed various techniques carrying out a pull-down release process first for detecting a generated erroneous interpolation in order to improve an image quality of a video image including the 2:2 or 2:3.
The technique noted in the Laid-Open Japanese Patent Application Publication No. 2005-102191 first releases a pull-down, then, if an erroneous interpolation is detected at the pull-down error detection unit, corrects the spot in which the erroneous interpolation has been detected by carrying out an interlace-use IP conversion. The employment of such a method makes it difficult to set a condition for checking an erroneous interpolation on one hand and precludes an effective use of information, which has been used for the conventional 2:2/2:3 detection process, in the process for detecting the erroneous interpolation on the other. Therefore, the production of such an image processing apparatus is relatively high cost. Furthermore, the process carried out at the pull-down error detection unit for detecting an erroneous interpolation can hardly be effective erroneous interpolation detection means because an erroneous interpolation is not necessarily judged numerically.
The technique noted in the Laid-Open Japanese Patent Application Publication No. 2004-180242 judges a telecine video image by detecting a repetition of the same image, thereby making it possible to respond to an erroneous interpolation due to a pull-down process. While the technique is effective to a video image in which a telecine video image (i.e., a video image generated by a telecine conversion) and an interlace video image are temporally intermixed; the technique, however, cannot respond to a video image in which they are spatially intermixed. Meanwhile, the technique noted in the Laid-Open Japanese Patent Application Publication No. 2004-180242 is one for responding to the problem of a video image signal being subjected to a pull-down process if the video image signal cannot be judged as a non-telecine image.
It is desirable to be capable of detecting the fact of an interlace IP conversion being performed for a video image signal including 2:2 and 2:3 and responding to an erroneous interpolation generated in association with performing the interlace-use IP conversion.